1. Field of the Invention
The present invention relates to a magnet apparatus for use in a magnetic resonance imaging system, and more particularly to a magnet apparatus having a magnetic shield.
2. Description of the Related Art
A magnet apparatus in use in a magnetic resonance imaging system (hereinafter referred to as the "MRI system") has a bore for receiving an object to be examined or a patient. A working volume in which a to-be-diagnosed portion of the object is located is defined in the bore. A static magnetic field or a main magnetic field is generated in the working volume by a main coil. A gradient magnetic field is superposed on the static magnetic field, and a high frequency signal is applied to the to-be-diagnosed portion of the object so that a tomographic image of the to-be-diagnosed portion is obtained.
The magnetic fluxes of the magnetic field leak from the bore to form a leaked magnetic field outside of the bore. The leaked magnetic field sometimes causes an adverse effect on the atmosphere around the MRI system. In order to eliminate this effect, a magnet apparatus is provided with a magnetic shield for reducing the leaked magnetic field. The magnetic shield is an active magnetic shield or cancel coils, for example.
The cancel coils generate a second magnetic field, the magnetic fluxes of which are directed in the direction opposite to that of the magnetic fluxes of the static magnetic field. The leaked magnetic field and the second magnetic field have substantially the same intensities. Therefore, both the magnetic fields cancel each other to reduce the leaked magnetic field greatly.
In the conventional magnet apparatus, cancel coils coaxially surround the main coil. A first bobbin of the main coil and a second bobbin of the cancel coils coaxially surrounding the first coil are used in order to arrange both coil like this. Since both coils have complicated structures, it is difficult to assemble them together. The axis of the second bobbin is sometimes deviated from that of the first bobbin due to an assembly error between both the coils and/or due to an electromagnetic force produced when the coils are excited. This causes the cancel coils to make misalignment with the main coil, with the result that a uniform static magnetic field is not maintained in the working volume and the tomographic image is deteriorated. Further, since the cancel coils are arranged radially outside of the main coil, the outer diameter of the magnet apparatus is rendered relatively large, making it difficult to transport the magnet apparatus.